There is increasing evidence that in experimental animals, moderate, local hyperthermia leads to a regression - and in some cases even the disappearance - of transplanted tumors without causing any damage to surrounding tissues. The aim of the work proposed is the development of a clinically useful system for producing controllable levels of hyperthermia sustained for periods of hours, restricted to the target tissue irrespective of its location in the human body. The use of one or more beams of ultrasound (unfocused, weakly focused or focused) with intensity modulation and/or controlled 3-dimensional spatial manipulation is considered the method of choice and will be evaluated. The effects of the ultrasonic irradiation procedure on normal tissue surrounding the target volume will be studied to establish its safety. The design of the system will permit its use in tumors of different shapes and at all clinically important sites in the body, simultaneously with X-radiation. Thus it should prove to be an extremely useful tool either alone or as an adjunct in radiation therapy of inoperable tumors whether its synergism is direct or through the oxygen effect from increased blood flow due to local hyperthermia.